Auto merge of #107044 - cuviper:more-llvm-ci, r=Mark-Simulacrum

ci: add runners for vanilla LLVM 14 and 15

As discussed in [zulip #t-infra][1] -- r? Mark-Simulacrum

[1]: https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/testing.20multiple.20llvm.20versions

Author: bors